1. Field of the Invention
The present invention relates to a liquid crystal display and particularly to a polymeric protective layer on the liquid crystal display. More particularly, the present invention relates to a protective layer formed from a cellulose ester having an inherent viscosity.(IV) of about 1.0 dl/g to less than about 2.0 dl/g.
2. Background of the Invention
Cellulose ester films and more particularly, a cellulose acetate film is used in various photographic or optical elements because it is tough and has flame retardant properties. Generally, cellulose acetate film has wide acceptance as a photographic support material. For example, U.S. Pat. Nos. 3,705,148 and 3,718,728 describe methods for improving the resistance to distortion and shrinkage of a cellulose acetate film when exposed to very high temperatures.
Recently, cellulose acetate film has been used in the manufacture of liquid crystal displays or LCDs due to the film having an optical isotropy. The cellulose acetate film is used as a protective film of a polarizing plate or a color filter in the liquid crystal display device. Generally, a liquid crystal display device includes an electrode disposed in each pixel for orienting the position of the liquid crystal molecules of a liquid crystal sealed in the device. The liquid crystal controls transmission of light there through according to the voltage generated by the electrodes in the pixels.
In such a conventional liquid crystal device, an active type thin film transistor is used. This conventional TFT LCD (Thin Film Transistor Liquid Crystal Display) device includes TFTs and pixel electrodes arranged on a TFT panel or bottom plate, a color filter for displaying color and a common electrode which are disposed on a color filter panel or top plate, a liquid crystal (LC) injected between the top and bottom plates, and a pair of polarizers disposed on the outer surfaces of the top and bottom plates for selectively transmitting light.
In the above conventional LCD, the device is designed to transmit light that passes through only the pixel electrodes and color filter and to cut off any other light. To do so, the conventional LCD device uses a light shielding layer (black matrix) formed in the color filter panel (top plate). However, in such cases, it is necessary to provide the black matrix formed on the top plate with extra margins to properly cover the TFT areas on the bottom plate. As a result, a larger black matrix occupying more space is required. This decreases the aperture ratio of the device.
In the production of organic derivatives of cellulose, and especially organic esters of cellulose, such as cellulose acetate, cellulose formate, cellulose propionate and cellulose butyrate, the esterification of cellulose with an organic acid results in a solution of the derivative of cellulose in an acid solvent. For example, in making cellulose acetate, cellulose is acetylated by contacting a cellulosic material, such as wood pulp, cotton linter, and the like with acetic anhydride and a catalyst in the presence of a relatively large amount of acetic acid. The acetic acid dissolves the cellulose acetate that is formed, producing a very heavy and viscous solution, referred to herein as the xe2x80x9cacid dopexe2x80x9d. Usually after hydrolysis, this solution of cellulose acetate is precipitated by adding water until the concentration of the acid reaches a point below which the acid will not hold the cellulose acetate in solution.
The isolation of cellulose acetate, or secondary cellulose materials, from organic solvent solutions, generally referred to as xe2x80x9csolvent dopexe2x80x9d or xe2x80x9cdopexe2x80x9d, has been extensively investigated. The known process for preparing cellulose acetate, i.e., a cellulose acetate with an approximate average degree of substitution with its acetylation and hydrolysis steps, results in a solution of the acetate in an acetic acid and water mixture. The acetic acid content and the polymerization degree (which has a correlation with the viscosity) of cellulose acetate influence the mechanical strength and the durability of a film obtained from the cellulose acetate. The elasticity, folding endurance, dimensional stability and resistance to moisture and heat decrease with decreasing the acetic acid content and the polymerization degree. An acetic acid content of 58% or more (preferably 59% or more) is necessary to satisfy the required quality of the photographic support or the optical film. The cellulose acetate having an acetic acid content of 58% or more is referred to as triacetyl cellulose. With respect to the polymerization degree, cellulose acetate preferably has a viscosity average degree of polymerization of not less than 270, and more preferably of not less than 290.
A cellulose ester film can be formed using either a melt casting method or a solvent casting method. The melt casting method includes heating the cellulose ester, which may optionally include a plasticizer, to form a melt, casting the melt on a support and cooling the melt to form a film. The film is then removed from the support.
In the solvent casting method, the cellulose ester is dissolved using a solvent to form the dope, casting the dope on a support and drying the dope to form a film. Optionally, the dope may include a plasticizer so that when the film is cast the plasticizer will be incorporated into the resultant film. The solvent cast method is capable of forming a highly flat film as compared to a film made using the melt cast method. Thus, the solvent cast method is generally employed to give a cellulose acetate film.
For various intermediate and end uses, high acetyl content cellulose acetate products are generally dissolved in volatile organic solvents such as dichloromethane and methanol. The solutions can be placed on objects so that when the solvent evaporates, a thin film or coating of cellulose acetate remains on the object. Thus, the solvent used in the solvent cast method must have functions not only of dissolving the cellulose acetate but also of forming an excellent film. In more detail, the viscosity and the polymer concentration of the dope should be appropriately adjusted to form a flat plane film having a uniform thickness. The dope also should have enough stability. Further, the dope should easily be set to gel. Furthermore, the formed film should easily be peeled off the support. The most appropriate solvent must be selected to satisfy these requirements. Moreover, the solvent should be so easily evaporated that the solvent scarcely can remain in the film.
Cellulose acetate films were originally developed for photographic films. For this use, they have certain requirements for mechanical strength and durability. The requirement that photographic film have the strength to survive photographic equipment such as cameras and projectors led to a high strength requirement. The need for sprocket holes in the film and the need for these holes to withstand the mechanical stress required the tensile strength of the film to be high. The intrinsic viscosity for photographic film quality cellulose acetate is about 2.0 dl/g. The intrinsic viscosity and degree of polymerization requirement has a significant effect on the cost and quality of the cellulose acetate film. In the cellulose acetate manufacturing step, higher degrees of polymerization require lower reaction temperatures, longer reaction times and produce less material from equipment. The higher degree of polymerization also requires lower catalyst levels that may lead to lower quality as shown by more insoluble material. In film casting, higher degree of polymerization requires lower solids levels in the casting dope and more filtration equipment and produces less material from a given set of equipment. Liquid crystal display applications use the same commercially available cellulose acetate as photographic film. However, the mechanical strength requirements for cellulose acetate used in a liquid crystal display are less than that for photographic film.
Accordingly, there is a need for a cellulose ester having sufficient strength for use in a LCD that can be made quicker and less expensive relative to a cellulose ester for use in photographic applications.
The present invention provides an improved LCD having a protective layer comprising a cellulose ester wherein the cellulose ester has a IV of from about 1.0 dl/g to less than about 2.0 dl/g.
It is an object of the present invention to provide an improved LCD protective layer wherein the protective layer includes a cellulose ester, and preferably includes cellulose acetate.